What effect does a potion of giant strength have upon a monster?

Question

An adventure module I own indicates that as soon as a fight breaks out, the villian's quasit minion will drink a potion of hill giant strength.

Looking at the quasit stat block, I am assuming its claw(or bite) attack is not strength based since the quasit has a strength of 5 (-3) but its claw(or bite) attack is +4 to hit. The damage bonus of the claw(or bite) attack is +3 (which matches its Dex of 17 (+3)). The block also states a quasit has a proficiency of +2.

A potion of hill giant strength grants a strength of 21(+5) for an hour. Should I just tweak the to-hit value to +6 and the damage bonus to +5 ??

I'm not sure if the to-hit number is supposed to make sense based on the Str(or Dex) of a monster and its proficiency score, but if it is, then is the quasit stat block wrong?

Answer 1

It influences the Strength bonus for to hit and damage

For modifying or designing monsters there is guidance in the DMG starting on page 273. The guidance is that you either can start with an attack bonus based on challenge rating, as listed on page 274, and tweak that, or that you can calculate it as you would a character attack bonus (page 277):

The table provides the baseline attack bonus for each challenge rating. Feel free to adjust the attack bonus as you see fit to match whatever concept you have in mind. (…)

Calculate Attack Bonuses. Alternatively, you can calculate a monster's attack bonuses the same way players calculate the attack bonuses of a character.
When a monster has an action that requires an attack roll, its attack bonus is equal to its proficiency bonus + its Strength or Dexterity modifier. A monster usually applies its Strength modifier to melee attacks and its Dexterity modifier to ranged attacks, although smaller monsters sometimes use Dexterity for both.

So there is no hard rule how to calculate it. That said, many if not most monsters in the Monster Manual follow exactly the logic of proficiency bonus plus Ability bonus, even if the imp does not (it would need to have +5 to hit with 17 Dexterity).

Because the imp is tiny, it normally uses its Dexterity bonus also for melee attacks instead of its Strength. But nothing stops it from using Strength instead, if that is better.

If it has consumed a potion of giant strength for a +5 Strength bonus, as you also can attack with Strength bonus in melee, it would make sense for the imp to use the better attack and damage bonus from Strength. The minimal proficiency bonus is +2, so that would give it +7 to hit and +5 to damage, if you use the method of calculating the scores like for a character.

Answer 2

I would just reverse engineer the creature in a reasonable way.

The Claw could be strength, dex or maybe something else based.

If it is strength based, it is -3 str+2 prof+5 other to hit.

If it is dex based, it is +3 dex+2 prof-1 other to hit.

The damage is +3, which makes me think it is dex based.

So my reverse engineering is:

Claws: Natural melee weapon. -1 to hit for 1d4 damage, finesse. the target must succeed on a DC 8+Proficiency+Charisma bonus Constitution saving throw or take 5 (2d4) poison damage and become poisoned for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

It then chooses to use Dex. We get +4 to hit, 1d4+3 damage, and DC 10.

We then give the Quasit to have +5 strength. It then chooses to use strength to attack with the claws.

We get +6 to hit (not +7!) and 1d4+5 damage. DC remains 10.

...

You can also do this without fully reverse engineering. Guess what attribute was being used, then calculate the difference in bonus to the new attribute used. So +3 dex becomes +5 strength -- a +2.

So we get +2 to hit and +2 to damage from the potion on an attack that was dex-based.